New name UI_UPDATE for interpret_move's return "".

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.

1 files changed, 2 insertions, 2 deletions

diff --git a/net.c b/net.c
index f4a1e39..fc603d1 100644
--- a/net.c
+++ b/net.c
@@ -2096,7 +2096,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
 
 	if (ui->cur_visible) {
 	    ui->cur_visible = FALSE;
-	    nullret = "";
+	    nullret = UI_UPDATE;
 	}
 
 	/*
@@ -2336,7 +2336,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
             OFFSET(ui->cur_x, ui->cur_y, ui->cur_x, ui->cur_y, dir, state);
             ui->cur_visible = TRUE;
         }
-        return "";
+        return UI_UPDATE;
     } else {
 	return NULL;
     }